Cross-reference and incorporation by reference is made to the following copending and commonly assigned U.S. patent applications: U.S. Appln. No. 09/448,987, filed Nov. 24, 1999, Attorney Docket No. D/99511Q, now U.S. Pat. No. 6,351,308, issued Feb. 26, 2002, and U.S. Appln. No. 09/449,263, filed Nov. 24, 1999, Attorney Docket No. D/99511Q1, both by Lingappa K. Mestha; Continuation-In-Part Appln. No. 09/535,007, filed Mar. 23, 2000, by the same Fred F. Hubble, III and Joel A. Kubby, Attorney Docket No. D/99511i, now U.S. Pat. No. 6,384,918 issued May 7, 2002, claiming priority from U.S. Appln. No. 09/448,774, filed Nov. 24, 1999, Attorney Docket No. D/99511 (abandoned); and Appln. No. 09/862,247, filed May 22, 2001: by Jagdish C. Tandon and Lingappa K. Mestha, Attorney Docket No. D/99660.
Disclosed in the embodiments herein is an improved, low cost, color spectrophotometer for color calibration or correction systems, highly suitable to be used for, or incorporated into, the color calibration or control of various color printing systems or other on-line color control or color processing systems.
In particular, the disclosed spectrophotometer provides improved accuracy for color test surfaces which may be tilted, curved, non-planer or otherwise varying in angular orientation relative to the spectrophotometer. That is particularly desirable for enhancement of the accuracy of a non-contact spectrophotometer which can measure the colors or color densities of test surfaces spaced from the spectrophotometer, especially moving surfaces, such as printed paper sheets in the output path of a color printer, or toner or liquid ink test patches on a moving photoreceptor or other surface, or other moving color objects, webs or materials, without requiring the test surface material to be constrained against a reference surface or against the spectrophotometer.
This freedom of movement of the test material, both transversely to, and variably spaced from, the spectrophotometer, can be quite desirable in various color measurement applications, such as allowing printed sheets to move freely between relatively widely spaced apart paper path baffles or guides. However, that freedom of movement can also allow the color test surface material or media to have variances in angular alignment relative to the spectrophotometer, which may be manifested by lead and trail edge curl or buckle induced by the media handling apparatus, or otherwise. The disclosed embodiments can desirably reduce measurement and output signal errors from such variances in angular alignment. There can also be azimuthal or media rotation measurement variances from differences in media reflectivity such as paper fiber orientation.
As will be further described, the improved angular and azimuthal insensitivity of the embodiment herein is fully compatible with, and is shown herein combinable with, various of the spectrophotometer features of the above-cross-referenced copending commonly owned U.S. Applications, especially the displacement insensitivity system of the above cross-referenced copending commonly owned U.S. application Ser. No. 09/535,007, filed Mar. 23, 2000, by the same Fred F. Hubble, III and Joel A. Kubby. Also it is optionally compatible with the multiple different photo-sites detectors of the above cross-referenced, commonly owned and contemporaneously filed Application by Jagdish C. Tandon and Lingappa K. Mestha, Attorney Docket No. D/99660 (also described herein, as an alternative embodiment).
In the specific exemplary spectrophotometer embodiment further described below, it may be seen that plural different color emission LEDs sequentially project their respective illuminations substantially in parallel, perpendicularly to the color test target surface, rather than at an angle thereto, so as to provide a substantially circular, rather than elliptical, commonly illuminated area of the test target. That is, with all the LEDs centrally located together, their illumination pattern on the test target may be formed from rays that hit the target at approximately 90 degrees, i.e., normal to the target. This will produce a circular or nearly circular irradiance pattern on a selected area of the target when the target surface is at 90 degrees thereto. One or more photo-sensors may be optically oriented at 45 degrees to the test target to receive the reflected light from the illuminated test target. As will be further explained herein, when the test target surface deviates from said 90 degrees, by factors such as paper lead or trail edge curl, paper buckle or corrugation, sensor mounting misalignment, or other effects, this circular LED irradiance pattern becomes only slightly elliptical, with little area change, and thus causes little variance in the target irradiance and, therefore, in the signals from the photo-sensors, hence providing improved spectrophotometer angular insensitivity.
An additional feature for improved spectrophotometer accuracy in the disclosed embodiment is to provide averaging of the outputs of plural photodetectors which are angularly viewing the target irradiance area from different positions around it, on opposing sides, so as to average any varying angular and/or azimuthal reflectivity of the target area, and thus further increase the insensitivity to changes in alignment with the target area.
Although not limited thereto, the exemplary spectrophotometer of the embodiments is shown and described herein as an integral part of an automatic on-line continuous color correction system of a color printer. That is because such a low cost spectrophotometer may be affordably provided and easily mounted in the output paths of color printers for automatic measurement of automatically printed color test sheets, without any manual effort or intervention being required, and without interfering with normal printing or the normal movement of printed sheets in the output path of the printer. Such color control systems are further described in the above and below cited co-pending applications and patents. For example, in Xerox Corp. U.S. Pat. No. 6,178,007 B1, issued Jan. 23, 2001, based on U.S. application Ser. No. 08/786,010, filed Jan. 21, 1997 by Steven J. Harrington, Attorney Docket No. D/96644, entitled xe2x80x9cMethod For Continuous Incremental Color Calibration For Color Document Output Terminals.xe2x80x9d The European patent application equivalent thereof was published by the European Patent Office on Jul. 22, 1998 as EPO publication No. 0 854 638 A2. Also, Xerox Corp. U.S. Pat. No. 6,222,648, issued Apr. 24, 2001, based on U.S. application Ser. No. 08/787,524, also filed Jan. 21, 1997, by Barry Wolf, et al, entitled xe2x80x9cOn Line Compensation for Slow Drift of Color Fidelity in Document Output Terminals (DOT),xe2x80x9d Attorney Docket No. D/96459. Also noted are Xerox Corp. U.S. Pat. No. 6,157,469, issued Dec. 5, 2000 and filed May 22, 1998, by Lingappa K. Mestha; Apple Computer, Inc. U.S. Pat. No. 5,881,209, issued Mar. 9, 1999; U.S. Pat. No. 5,612,902, issued Mar. 18, 1997 to Michael Stokes, and other patents and applications further noted below.
A low cost, relatively simple, spectrophotometer, as disclosed herein, is thus particularly (but not exclusively) highly desirable for such a xe2x80x9ccolorimetryxe2x80x9d function for such an on-line printer color correction system. Where at least one dedicated spectrophotometer is being provided in each printer, its cost and other factors becomes much more significant, as compared to the high cost (and other unsuitability""s for on-line use) of typical laboratory spectrophotometers.
An early patent of interest as to suggesting colorimetry in the printed sheets output of a color printer is Xerox Corp. U.S. Pat. No. 5,748,221, issued May 5, 1998 to Vittorio Castelli, et al, filed Nov. 1, 1995 (D/95398). This patent is also of particular interest here for its Col. 6, lines 18 to 28 description of measuring color:
xe2x80x9c . . . by imaging a part of an illuminated color patch on three amorphous silicon detector elements after filtering with red, green and blue materials. The technology is akin to that of color input scanners. The detector outputs can be used as densitometric values to assure color consistency. Calibration of the resulting instrument outputs against measurement by laboratory colorimeters taken over a large sample of patches made by the toners of the printer of interest allows mapping to absolute color coordinates (such as L*a*b*).xe2x80x9d
As disclosed in the cited references, automatic on-line color recalibration systems can be much more effective with an on-line color measurement system where a spectrophotometer may be mounted in the paper path of the moving copy sheets in the printer, preferably in the output path after fusing or drying, without having to otherwise modify the printer, or interfere with or interrupt normal printing, or the movement of the printed sheets in said paper path, and yet provide accurate color measurements of test color patches printed on the moving sheets as they pass the spectrophotometer. That enables a complete closed loop color control of a printer.
However, it should be noted that color measurements, and/or the use of color measurements for various quality or consistency control functions, are also important for many other different technologies and applications, such as in the production of textiles, wallpaper, plastics, paint, inks, etc. Thus, the disclosed color detection system may have applications in various such other fields where these materials or objects are to be color tested. Although the specific exemplary embodiment herein is part of a preferred automatic recalibration system with an on-line color printer color spectrophotometer, it will be appreciated that the disclosed spectrophotometer is not limited to that disclosed application.
By way of general background, studies have demonstrated that humans are particularly sensitive to spatial color variations. Typical full color printing controls, as well as typical color controls in other commercial industries, still typically utilize manual off-line color testing and frequent manual color adjustments by skilled operators. Both the cost and the difficulty of on-line use of prior color measurement apparatus and control systems, and the need for manual recalibration steps, has heretofore inhibited automation of many of such various commercial color testing and color adjustment systems. The disclosed lower cost spectrophotometer addresses both of those concerns.
By way of some examples of the construction or design of various other color spectrophotometers themselves, besides Xerox Corp. U.S. Pat. No. 5,748,221 above, and, especially, the above cross-referenced U.S. application Ser. No. 09/535,007, filed Mar. 23, 2000, by Fred F. Hubble, III and Joel A. Kubby, there is noted HP U.S. Pat. No. 5,671,059, issued 1993; and HP U.S. Pat. No. 5,272,518, issued Dec. 21, 1993; Accuracy Microsensor, Inc. U.S. Pat. Nos. 5,838,451 and 5,137,364, both issued to Cornelius J. McCarthy on Nov. 17, 1998 and Aug. 11, 1992, respectively; Color Savvy U.S. Pat. Nos. 6,147,761, 6,020,583, and 5,963,333; BYK-Gardner U.S. Pat. No. 5,844,680; and Colorimeter U.S. Pat. No. 6,157,454.
Some patents of interest relating to densitometers include U.S. Pat. Nos. 4,553,033; 4,989,985; and 5,078,497. Patents relating to providing uniform surface irradiance include U.S. Pat. No. 5,526,190.
As used in the patent claims and elsewhere herein, unless otherwise specifically indicated, the term xe2x80x9cspectrophotometerxe2x80x9d may encompass a spectrophotometer, colorimeter, and densitometer, as broadly defined herein. That is, the word xe2x80x9cspectrophotometerxe2x80x9d may be given the broadest possible definition and coverage in the claims herein, consistent with the rest of the claim. For example, various of the claims to a xe2x80x9cspectrophotometerxe2x80x9d in this application may encompass a similarly improved ETAC sensor densitometer used to measure the black or other color toner development on a moving photoreceptor surface or transfer belt. The definition or use of such above technical terms may also vary or differ among various scientists and engineers.
The following is an attempt to provide some simplified clarifications relating and distinguishing the respective terms xe2x80x9cspectrophotometer,xe2x80x9d xe2x80x9ccolorimeter,xe2x80x9d and xe2x80x9cdensitometer,xe2x80x9d as they may be used in the specific context of specification embodiment examples of providing components for an on-line color printer color correction system, but not necessarily as claim limitations.
A typical xe2x80x9cspectrophotometerxe2x80x9d measures the reflectance of an illuminated object of interest over many light wavelengths. Typical prior spectrophotometers in this context use 16 or 32 channels measuring from 400 nm to 700 nm or so, to cover the humanly visible color spectra or wavelength range. A typical spectrophotometer gives color information in terms of measured reflectances or transmittances of light, at the different wavelengths of light, from the test surface. (This is to measure more closely to what the human eye would see as a combined image of a broad white light spectra image reflectance, but the spectrophotometer desirably provides distinct electrical signals corresponding to the different levels of reflected light from the respective different illumination wavelength ranges or channels.)
A xe2x80x9ccalorimeterxe2x80x9d normally has three illumination channels, red, green and blue. That is, generally, a xe2x80x9ccolorimeterxe2x80x9d provides its three (red, green and blue, or xe2x80x9cRGBxe2x80x9d) values as read by a light sensor or detector receiving reflected light from a color test surface sequentially illuminated with red, green and blue illuminators, such as three different color LEDs or three lamps with three different color filters. It may thus be considered different from, or a limited special case of, a xe2x80x9cspectrophotometerxe2x80x9d, in that it provides output color information in the trichromatic quantity known as RGB.
Trichromatic quantities may be used for representing color in three coordinate space through some type of transformation. Other RGB conversions to xe2x80x9cdevice independent color spacexe2x80x9d (i.e., RGB converted to conventional L*a*b*) typically use a color conversion transformation equation, or a xe2x80x9clookup tablexe2x80x9d system, in a known manner. (Examples are provided in references cited herein, and elsewhere.)
A xe2x80x9cdensitometerxe2x80x9d typically has only a single channel, and simply measures the amplitude of light reflectivity from the test surface, such as a developed toner test patch on a photoreceptor, at a selected angle over a range of wavelengths, which may be wide or narrow. A single illumination source, such as an IR LED, a visible LED, or an incandescent lamp may be used. The output of the densitometer detector is programmed to give the optical density of the sample. A densitometer of this type is basically xe2x80x9ccolor blindxe2x80x9d. For example, a cyan test patch and magenta test patch could have the same optical densities as seen by the densitometer, but, of course, exhibit different colors.
A multiple LED reflectance spectrophotometer, as in the specific examples of the embodiments herein, may be considered to belong to a special case of spectrophotometers which normally illuminate the target with narrow band or monochromatic light. Others, with wide band illumination sources, can be flashed Xenon lamp spectrophotometers, or incandescent lamp spectrophotometers. A spectrophotometer is normally programmed to give more detailed, and broader spectra, reflectance values by using more than 3 channel measurements (e.g., 10 or more channel measurements), with conversion algorithms. That is in contrast to normal three channel calorimeters, which cannot give accurate, human eye related, reflectance spectra measurements, because they have insufficient measurements for that (only three measurements).
The spectrophotometer of the disclosed embodiments is especially suitable for being mounted at one side of the printed sheets output path of a color printer to optically evaluate color imprinted output sheets as they move past the spectrophotometer, variably spaced therefrom, without having to contact the sheets or interfere with the normal movement of the sheets. In particular, it may be used to measure a limited number of color test patch samples printed by the printer on actual printed sheet output of the printer during regular or selected printer operation intervals (between normal printing runs or print jobs). These color test sheet printing intervals may be at regular timed intervals, and/or at each machine xe2x80x9ccycle-up,xe2x80x9d or as otherwise directed by the system software. The spectrophotometer may be mounted at one side of the paper path of the machine, or, if it is desired to use duplex color test sheets, two spectrophotometers may be mounted on opposite sides of the paper path.
Relatively frequent color recalibration of a color printer is highly desirable, since the colors actually printed on the output media (as compared to the colors intended to be printed) can significantly change, or drift out of calibration over time, for various known reasons. For example, changes in the selected or loaded print media, such as differences paper or plastic sheet types, materials, weights, calendaring, coating, humidity, etc. Or changes in the printer""s ambient conditions, changes in the image developer materials, aging or wear of printer components, varying interactions of different colors being printed, etc. Printing test color patches on test sheets of the same print media under the same printing conditions during the same relative time periods as the color print job being color-controlled is thus very desirable.
It is thus also advantageous to provide dual-mode color test sheets, in which multiple color patches of different colors are printed on otherwise blank areas of each, or selected, banner, cover, or other inter-document or print job separator sheets. Different sets of colors may be printed on different banner or other test sheets. This dual use of such sheets saves both print paper and printer utilization time, and also provides frequent color recalibration opportunities where the printing system is one in which banner sheets are being printed at frequent intervals anyway.
An additional feature which can be provided is to tailor or set the particular colors or combinations of the test patches on a particular banner or other test sheet to those colors which are about to be printed on the specific document for that banner sheet, i.e., the document pages which are to be printed immediately subsequent to that banner sheet (the print job identified by that banner sheet). This can provide a xe2x80x9creal timexe2x80x9d color correction for the color printer which is tailored to correct printing of the colors of the very next document to be printed.
The preferred implementations of the systems and features disclosed herein may vary depending on the situation. Also, various of the disclosed features or components may be alternatively used for such functions as gray scale balancing, turning on more than one illumination source at once.
It will be appreciated that these test patch images and colors may be automatically sent to the printer imager from a stored data file specifically designed for printing the dual mode banner sheet or other color test sheet page, and/or they may be embedded inside the customer job containing the banner page. That is, the latter may be directly electronically associated with the electronic document to be printed, and/or generated or transmitted by the document author or sender. Because the printed test sheet color patches colors and their printing sequence is known (and stored) information, the on-line spectrophotometer measurement data therefrom can be automatically coordinated and compared.
After the spectrophotometer or other color sensor reads the colors of the test patches, the measured color signals may be automatically processed inside the system controller or the printer controller to produce or modify the tone reproduction curve, as explained in the cited references. The color test patches on the next test sheet may then be printed with that new tone reproduction curve. This process may be repeated so as to generate further corrected tone reproduction curves. If the printer""s color image printing components and materials are relatively stable, with only relatively slow long term drift, and there is not a print media or other abrupt change, the tone reproduction curve produced using this closed loop control system will be the correct curve for achieving consistent colors for at least one or even a substantial number of customer print jobs printed thereafter, and only relatively infrequent and few color test sheets, such as the normal banner sheets, need be printed.
However, If there are substantial changes in the print media being used by the printer, or other sudden and major disturbances in the printed colors (which can be detected by the spectrophotometer output in response to the test patches on the next dual mode banner sheet or other color test sheet, or even, in certain instances, in the imprinted images) then the subsequent customer print job may have incorrect color reproduction. In these situations of customer print media changes in the printer (or new print jobs or job tickets that specify a change in print media for that print job), where that print media change is such that it may substantially affect the accuracy of the printed colors for that subsequent print job, it is not desirable to continue printing and then have to discard the next subsequent print jobs printed with customer unacceptable colors. In that situation it may be preferable in color critical applications to interrupt the normal printing sequence once the sudden color printing disturbance is detected and to instead print plural additional color test sheets in immediate succession, with different color test patch colors, to sense and converge on a new tone reproduction curve that will achieve consistent color printing for that new print media, and only then to resume the normal printing sequence of customer print jobs. Thus, the subsequent customer print jobs would then use the final, re-stabilized, tone reproduction curve obtained after such a predetermined number of sequential plural color test sheets have been printed.
This patent application is not related to or limited to any particular one of the various possible (see, e.g., various of the cited references) algorithms or mathematical techniques for processing the electronic signals from the spectrophotometer to generate or update color correction tables, tone reproduction curves, or other color controls, and hence they need not be further discussed herein.
Various possible color correction systems can employ the output signals of spectrophotometers, using various sophisticated feedback, correction and calibration systems, which need not be discussed in any further detail here, since the general concepts and many specific embodiments are disclosed in many other patents (including those cited herein) and publications. In particular, to electronically analyze and utilize the spectrophotometer or other electronic printed color output information with a feedback analysis system for the color control systems for a printer or other color reproduction system. It is, however, desirable in such systems to be able to use a substantially reduced (smaller) number of color patch samples, printed at intervals during the regular printing operations, to provide relatively substantially continuous updating correction of the printer""s color renditions over a wide or substantially complete color spectra. Noted especially in that regard is the above-cited Xerox Corp. Steven J. Harrington U.S. Pat. No. 6,178,007 B1.
Color correction and/or color control systems should not be confused with color registration systems or sensors. Those systems are for insuring that colors are correctly printed accurately superposed and/or accurately adjacent to one another, such as by providing positional information for shifting the position of respective color images being printed.
Other background patents which have been cited as to color control or correction systems for printers include the following U.S. patents: Xerox Corp. U.S. Pat. No. 5,963,244, issued Oct. 5, 1999 to L. K. Mestha, et al, entitled xe2x80x9cOptimal Reconstruction of Tone Reproduction Curvexe2x80x9d (using a lookup table and densitometer readings of photoreceptor sample color test patches to control various color printer parameters); U.S. Pat. No. 5,581,376, issued December 1996 to Harrington; U.S. Pat. No. 5,528,386, issued Jun. 18, 1996 to Rolleston et al.; U.S. Pat. No. 4,275,413, issued Jun. 23, 1981 to Sakamoto et al.; U.S. Pat. No. 4,500,919, issued Feb. 19, 1985 to Schreiber; U.S. Pat. No. 5,416,613, issued May 16, 1995 to Rolleston et al.; U.S. Pat. No. 5,508,826, filed Apr. 27, 1993 and issued Apr. 16, 1996 to William J. Lloyd et al.; U.S. Pat. No. 5,471,324, issued Nov. 28, 1995 to Rolleston; U.S. Pat. No. 5,491,568, issued Feb. 13, 1996 to Wan; U.S. Pat. No. 5,539,522, issued Jul. 23, 1996 to Yoshida; U.S. Pat. No. 5,483,360, issued Jan. 9, 1996 to Rolleston et al.; U.S. Pat. No. 5,594,557, issued January 1997 to Rolleston et al.; U.S. Pat. No. 2,790,844 issued April 1957 to Neugebauer; U.S. Pat. No. 4,500,919, issued February 1985 to Schreiber; U.S. Pat. No. 5,491,568, issued Feb. 13, 1996 to Wan; U.S. Pat. No. 5,481,380 to Bestmann, issued Jan. 2, 1996; U.S. Pat. No. 5,664,072, issued Sep. 2, 1997 to Ueda et al.; U.S. Pat. No. 5,544,258, issued Aug. 6, 1996 to Levien; and U.S. Pat. No. 5,881,209, filed Sep. 13, 1994 and issued Mar. 9, 1999 to Michael Stokes.
By way of further background on the subject of technology for automatic color correction for color printers or other reproduction apparatus, especially such systems utilizing feedback signals from a colorimeter or spectrophotometer (as noted, those terms may be used interchangeably herein), and/or automatically measuring the actually printed colors of test patches on printed copy sheets as they are being fed through the output path the printer, there is noted the following: the above-cited Xerox Corp. U.S. Pat. No. 5,748,221, filed Nov. 1, 1995 and issued May 5, 1998 to V. Castelli, et al, entitled xe2x80x9cApparatus for Colorimetry, Gloss and Registration Feedback in a Color Printing Machine,xe2x80x9d (noting especially the calorimeter detector details); the above-cited Apple Computer, Inc. U.S. Pat. No. 5,612,902, issued Mar. 18, 1997 to Michael Stokes; Xerox Corp. U.S. Pat. No. 5,510,896, issued Apr. 23, 1996 to Walter Wafler, filed Jun. 18, 1993 (see especially Col. 8 re color calibration from information from a scanned color test copy sheet as compared to original color image information); and Xerox Corp. U.S. Pat. No. 5,884,118, issued Mar. 16, 1999 to Mantell and L. K. Mestha, et al, entitled xe2x80x9cPrinter Having Print Output Linked to Scanner Input for Automated Image Quality Adjustmentxe2x80x9d (note especially Col. 6 lines 45-49).
U.S. Patents of interest to color correction in general, but which may be useful with, or provide background information for, the above or other systems, include the above-cited Xerox Corp. U.S. Pat. No. 5,594,557, filed Oct. 3, 1994 and issued Jan. 14, 1997 to R. J. Rolleston et al., entitled xe2x80x9cColor Printer Calibration Correcting for Local Printer Non-Linearities;xe2x80x9d Seiko Epson Corp. U.S. Pat. No. 5,809,213, provisionally filed Feb. 23, 1996 and issued Sep. 15, 1998 to A. K. Bhattacharjya re reduced color measurement samples; and Splash Technology, Inc. U.S. Pat. No. 5,760,913 filed Feb. 12, 1996 and issued Jun. 2, 1998 to Richard A. Falk in which a calibration image is scanned using a scanner coupled to the printing system with a personal computer.
In addition to above-cited issued patents, also noted as of possible interest to on-line color printer color control or correction systems (other than spectrophotometers per se) are Xerox Corp. U.S. Applications including: U.S. application Ser. No. 09/083,202, filed May 22, 1998 by Mark A. Scheuer, et al., entitled xe2x80x9cDevice Independent Color Controller and Method,xe2x80x9d Attorney Docket No. D/97695; U.S. application Ser. No. 09/083,203, filed May 22, 1998 by Lingappa K. Mestha, entitled xe2x80x9cDynamic Device Independent Image,xe2x80x9d Attorney Docket No. D/98203 (now U.S. Pat. No. 6,157,469, issued Dec. 5, 2000); U.S. application Ser. No. 09/232,465, filed Jan. 19, 1999 by Martin E. Banton, et al., entitled xe2x80x9cApparatus and Method for Using Feedback and Feedforward in the Generation of Presentation Images in a Distributed Digital Image Processing System,xe2x80x9d Attorney Docket No. D/98423; U.S. application Ser. No. 09/221,996, filed Dec. 29, 1998 by Lingappa K. Mestha, et al., entitled xe2x80x9cColor Adjustment Apparatus and Method,xe2x80x9d Attorney Docket No. D/98428; U.S. application Ser. No. 09/455,761, filed Dec. 7, 1999 by Sidney W. Marshall, et al., entitled xe2x80x9cColor Gamut Mapping for Accurately Mapping Certain Critical Colors and Corresponding Transforming of Nearby Colors and Enhancing Global Smoothness,xe2x80x9d Attorney Docket No. D/99087; U.S. application Ser. No. 09/487,586, filed Jan. 19, 2000 by Lingappa K. Mestha, et al., entitled xe2x80x9cMethods For Producing Device and Illumination Independent Color Reproduction,xe2x80x9d Attorney Docket No. D/99159; U.S. application Ser. No. 09/451,215, filed Nov. 29, 1999 by Lingappa K. Mestha, et al., entitled xe2x80x9cOn-Line Model Prediction and Calibration System For A Dynamically Varying Color Marking Device,xe2x80x9d Attorney Docket No. D/99508; U.S. application Ser. No. 09/454,431, filed Dec. 3, 1999, by Tracy E. Thieret, et al., entitled xe2x80x9cOn-Line Piecewise Homemorphism Model Prediction, Control and Calibration System for a Dynamically Varying Color Marking Device,xe2x80x9d Attorney Docket No. D/99577Q; U.S. application Ser. No. 09/461,072, filed Dec. 15, 1999 by Lingappa K. Mestha, et al., entitled xe2x80x9cSystems and Methods for Device Independent Color Control to Achieve Accurate Color Proofing and Reproduction,xe2x80x9d Attorney Docket No. D/99627; U.S. application Ser. No. 09/562,072, filed May 1, 2000 by Lingappa K. Mestha, et al., entitled xe2x80x9cSystem and Method for Reconstruction of Spectral Curves, Using Measurements from a Color Sensor and Statistical Techniques,xe2x80x9d Attorney Docket No. D/99803; U.S. application Ser. No. 09/621,860, filed Jul. 21, 2000 by Lingappa K. Mestha, et al., entitled xe2x80x9cSystem and Method for Reconstruction of Spectral Curves Using Measurements from a Color Sensor and a Spectral Measurement System Model,xe2x80x9d Attorney Docket No. D/A0098; and U.S. application Ser. No. 09/566,291, filed May 5, 2000 by Lingappa K. Mestha, et al., entitled xe2x80x9cOn-Line Calibration System For A Dynamically Varying Color Marking Device,xe2x80x9d Attorney Docket No. D/A0102.
As further well-known background for on difficulties in color correction of printers in general, computers and other electronic equipment generating and inputting color images or documents typically generate three-dimensional or RGB (red, green, blue) color signals. These color signals may be transmitted as PDL or other device independent terms to a specific server or printer for a xe2x80x9cRIPxe2x80x9d (raster image process) conversion to device dependent color values, such as for the line and bit signals for the laser scanner or LED bar of the particular printer. Many printers, however, can receive four-dimensional or CMYK (cyan, magenta, yellow, and black) signals as input, and/or can print with four such print colors (although the printed images can still be measured as corresponding RGB values). A look-up table is commonly provided to convert each digital RGB color signal value to a corresponding digital CMYK value before or after being received by the printer.
Real-world printers inherently have non-ideal printing materials, colors and behaviors, and therefore have complex non-linear colorimetric responses. Also, interactions between the cyan, magenta, and yellow imaging materials exist, especially on the printed output, which result in unwanted or unintended absorptions and/or reflections of colors. Even after a printer is initially calibrated, such that one or a range of input digital CMYK values produce proper colors, the full spectrum of CMYK values and printed colors will not be or remain fully accurate. In other words, the colors requested or directed to be printed by various input signals will not be the same as the actual colors printed.
This discrepancy arises in part because the relationship between the digital input values that drive the printer and the resulting calorimetric response is a complex non-linear function. Labeling the response, or other values, as xe2x80x9ccolorimetricxe2x80x9d can indicate that the response or value has been measured by such an instrument. Adequately modeling the colorimetric response of a printer to achieve linearity across the entire available spectrum requires many parameters. Typically, a color correction look-up table is built which approximates the mapping between RGB colorimetric space and CMYK values, as taught in various of the above-cited references. Each RGB coordinate may be typically represented by an 8-bit red value, an 8-bit green value, and an 8-bit blue value. Although those RGB coordinates are capable of addressing a look-up table having 2563 locations, measuring and storing 2563 values is time consuming and expensive. The look-up table is thus typically partitioned into a smaller size such as 16xc3x9716xc3x9716 (4096) table locations, each of which stores a four-dimensional CMYK value. Other CMYK values may then be found by interpolating the known CMYK values using an interpolation process, for example, trilinear or tetrahedral interpolation.
The color correction look-up table may be built by sending a set of CMYK digital values to the printer, measuring the colorimetric RGB values of the resulting color patches outputted by the printer with a spectrophotometer, and generating the look-up table from the difference between the inputted values and the measured outputted values. More specifically, the color correction look-up table corrects for non-linearities, printing parameter variations, and unwanted absorptions of inks, so that the printer will print the true corresponding color.
After the color correction table is generated, the actual printer response may tend to drift over time. To correct for the drift, the system is adjusted or recalibrated periodically. Recalibrating the color correction table involves periodically printing and remeasuring a set of test color patches which are then compared to an original set of color patches by calibration software. Remeasuring, however, has heretofore more typically been performed manually by a scanner or other measuring device which is remote from the printer being recalibrated. For example, by removing a test output sheet from the printer output tray, placing it (stationary) on a table and sliding a spectrophotometer over it, manually or with an X-Y plotter driver, or automatically feeding the test sheet through the spectrophotometer, and storing the spectrophotometer output signals data in an associated memory to read out later, or, connecting the spectrophotometer by an electrical wire or cable to the printer controller or its server to directly receive those color recalibration electrical input signals from the spectrophotometer and process them as described. The connecting cable could be replaced by known IR or RF wireless (such as xe2x80x9cBlueToothxe2x80x9d) connection systems, as used in PC and other electronic components connections. However, this off-line manual testing of calibration sheets assumes that the operator can properly manually identify and measure the test color sheets or patches being tested in the correct order, from the correct machine. Once a color correction table is generated, it must be associated with the correct printer, otherwise, a different printer will be recalibrated with an incorrect correction table. An automatic, on-line, dedicated spectrophotometer color correction system does not have these problems or potential error sources.
It will be appreciated that although the specific embodiment herein is described with particular reference to such desirable applications for calibrating and/or regularly re-calibrating color printers and/or refining color correction tables, that what is disclosed herein may also find various other applications in other color testing and correction systems and industries.
As discussed, in high quality color reprographic applications, it is highly advantageous to monitor and update system calorimetric performance on-line and automatically through the use of an integrated spectrophotometer. That is, to have the printing device automatically fairly frequently generate calibration prints on otherwise normally printed sheets with color patches based on digital test pattern generations, and to have a spectrophotometer in the printer output which can read those moving sheet printed color test patches accurately to provide printed output color measurement signals, without manual intervention or printing interference. This requires a relatively low cost, yet fast, accurate, and wide spectral range spectrophotometer capable of effectively operating in that environment, and under those conditions, without interfering with normal printing operations. That is, being of sufficiently low cost such that this enhanced feature can be provided on commercial color printers without substantially increasing the total customer cost of those printers. That is not typical for conventional laboratory spectrophotometers. The disclosed spectrophotometer embodiment may be positioned at any convenient location along the normal paper path of a printing machine. It may even be fitted into the output sheet stacker tray of various existing color printers.
A specific feature of the specific embodiment disclosed herein is to provide a color correction system for a color printer having an output path for moving printed color sheets, including printed test sheets with printed color test patches, in which a spectrophotometer is mounted adjacent to said printer output path for sensing the colors printed on said printed color test patches on said printed test sheets as said printed test sheets are moving past said spectrophotometer in said output path, said color correction system including plural illumination sources for sequentially illuminating said color test patches with different illumination spectra in a substantially circular illumination pattern, and a plural photodetectors system for providing electrical output signals in response to the color of said test patches from said sequential illumination of said test patches by reflection of said illumination of said color test patches in said substantially circular illumination pattern by said illumination sources, and a plural lens system for transmitting said reflected illumination from said color test patches to said plural photodetectors system, so as to provide improved angular displacement insensitivity of said spectrophotometer relative to said printed test sheets.
Further specific features disclosed herein, individually or in combination, include those wherein said plural photodetectors system comprises a plurality of photodetectors angularly spaced around said substantially circular illumination pattern to receive reflected illumination therefrom at substantially the same angle substantially from opposing directions; and/or wherein a common central lens system is provided, and wherein said different illumination spectra from said plural illumination sources for sequentially illuminating said color test patches in a substantially circular illumination pattern are perpendicularly aimed by said central lens system at substantially the same area of said color test patches on said printed test sheets to provide said substantially circular illumination pattern thereof; and/or wherein said plural illumination sources comprise a plurality of closely adjacent different spectral emission LEDs with a sequential actuation circuit; and/or a low cost broad spectrum spectrophotometer for measuring the colors of test target areas which may be variably displaced and variably angularly oriented relative to said spectrophotometer, comprising plural illumination sources and a common lens system for sequentially projecting plural different spectral illuminations from said plural illumination sources substantially perpendicular to the color test target area to illuminate a substantially circular illumination area of the color test target area, a sequential actuation circuit for sequentially actuation of said plural illumination sources, and at least one photodetector spaced from said common lens system and spaced from said substantially circular illumination area of the color test target area to angularly receive reflected light from said substantially circular illumination area, so as to provide reduced angular displacement sensitivity of said spectrophotometer relative to said color test target area; and/or wherein said at least one photodetector comprises a detector array of plural photodetectors spaced apart on at least two opposing sides of said common lens system and spaced from said substantially circular illumination area of the color test target area to differently angularly receive reflected light from said substantially circular illumination area at substantially the same angle from opposing directions, so as to provide additionally reduced angular displacement sensitivity of said spectrophotometer relative to said color test target area; and/or wherein said plural illumination sources comprise approximately eight or less LEDs providing a correspondingly limited number of different spectral illuminations; and/or wherein said spectrophotometer is a part of a color control system of a color printer with a printed sheets output path and is mounted adjacent to at least one side of the printed sheets output path of said color printer and said illuminated color test target area is printed on a printed color test sheet printed by said printer and moving past said spectrophotometer in said printed sheets output path of said color printer; and/or wherein said limited plurality of illumination sources comprises less than approximately five LEDs providing a corresponding limited number of different spectral illuminations, and said at least one photodetector comprises a photodetector with plural photo-sites and plural different spectral responses; and/or wherein there are approximately four or less said plural photodetectors; and/or wherein said plural photodetectors are mounted in a substantially circular pattern surrounding said common lens system to define a central axis; and/or wherein said plural photodetectors each have a projection lens system having approximately 1:1 imaging optics; and/or a method of broad spectrum color measurement of a color test area comprising substantially perpendicularly sequentially illuminating a substantially circular area of said color test area with a limited plural number of different spectra illuminations through a common lens system and sequentially non-contact measuring a portion of the reflected illumination from said sequentially illuminated substantially circular area of said color test area at approximately 45 degrees thereto with at least one photodetector spaced from said color test area to provide reduced angular insensitivity of said photodetector relative to said color test area; and/or the method of broad spectrum color measurement of a color test area, utilizing a detector array comprising plural photodetectors mounted on opposing sides of said color test area, the outputs of which are averaged to reduce said angular insensitivity relative to said color test area; and/or wherein said plural photodetectors each receive said reflected illumination through a projection lens system having an approximately 1:1 imaging ratio to provide relative displacement insensitivity of said photodetectors relative to said color test area; and/or a low cost broad spectrum spectrophotometer comprising means for sequentially perpendicularly illuminating a common substantially circularly illuminated color test area with a limited plural number of different spectra illuminations, and means for sequentially measuring the angularly reflected illumination from said sequentially illuminated substantially circularly illuminated color test area by applying said reflected illumination simultaneously to plural photodetectors spaced around said common color test area and summing the outputs of said photodetectors to provide an averaged output; and/or wherein said limited plural number of different spectra illuminations is provided by a plurality of different LEDs in a common area with a common shared lens system and a sequential actuation circuit for said LEDs.
The disclosed system may be connected, operated and controlled by appropriate operation of conventional control systems. It is well-known and preferable to program and execute various control functions and logic with software instructions for conventional or general purpose microprocessors, as taught by numerous prior patents and commercial products. Such programming or software may of course vary depending on the particular functions, software type, and microprocessor or other computer system utilized, but will be available to, or readily programmable without undue experimentation from functional descriptions, such as those provided herein, and/or prior knowledge of functions which are conventional, together with general knowledge in the software and computer arts. Alternatively, the disclosed control system or method may be implemented partially or fully in hardware, using standard logic circuits or single chip VLSI designs.
In the description herein the term xe2x80x9csheetxe2x80x9d refers to a usually flimsy (non-rigid) physical sheet of paper, plastic, or other suitable physical substrate or print media for images, whether precut or web fed. A xe2x80x9ccopy sheetxe2x80x9d may be abbreviated as a xe2x80x9ccopy,xe2x80x9d or called a xe2x80x9chardcopy.xe2x80x9d Printed sheets may be referred to as the xe2x80x9coutput.xe2x80x9d A xe2x80x9cprint jobxe2x80x9d is normally a set of related printed sheets, usually one or more collated copy sets copied from a one or more original document sheets or electronic document page images, from a particular user, or otherwise related.
As to specific components of the subject apparatus, or alternatives therefor, it will be appreciated that, as is normally the case, some such components are known per se in other apparatus or applications which may be additionally or alternatively used herein, including those from art cited herein. All references cited in this specification, and their references, are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features, and/or technical background. What is well known to those skilled in the art need not be described here.